The invention relates to the field of transmitting digital data by optical means. It is more particularly concerned with transmission at high bit rates on long-haul fiber optic links.
More specifically, the invention is related to a frequency allocation scheme for a set of optical channels (each at specific carrier frequency) multiplexed using wavelength-division multiplexing and polarization-division multiplexing.
The invention is also related to a transmission system with a transmitter function, a transmitting fiber and a receiver function and the transmitter function comprises light sources, with modulators, possibly polarizers and a wavelength multiplexer and the receiver function comprises one or several polarization demultiplexers, a wavelength demultiplexer, filters and receivers.
Such a transmission scheme uses an optical transmitter connected to an optical receiver by the fiber link. The transmitter generally modulates the power of an optical carrier wave from a laser oscillator as a function of the information to be transmitted. NRZ or RZ modulation is very frequently used and entails varying the power of the carrier wave between two levels: a low level corresponding to extinction of the wave and a high level corresponding to a maximum optical power. The variations of level are triggered at times imposed by a clock rate and this defines successive time cells allocated to the binary data to be transmitted. By convention, the low and high levels respectively represent the binary values xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d.
The maximum transmission distance is generally limited by the ability of receivers to detect without error these two power levels after the modulated wave has propagated in the optical link. The usual way to increase this distance is to increase the ratio between the average optical power of the high levels and that of the low levels; this ratio defining the xe2x80x9cextinction ratioxe2x80x9d which is one of the characteristics of the modulation. For a given distance and a given extinction ratio, the information bit rate is limited by chromatic dispersion generated in the fibers. This dispersion results from the effective index of the fiber depending on the wavelength of the wave transported, and it has the consequence that the width of the transmitted pulses increases as they propagate along the fiber. This phenomenon is characterized by the dispersion coefficient D of the fiber, which is defined as a function of the propagation constant xcex2 by the equation D=xe2x88x92(2xcfx80c/xcex2)d2xcex2/dxcfx892, where xcex and xcfx89 are respectively the wavelength and the angular frequency of the wave.
Not only does chromatic dispersion limit the possibility of transmission, but also it is a main factor for distortion. Increasing the data rate up to higher levels about T Bit/sxe2x80x94the effects of the fibers increase the impact on the received signal. One solution is the use of DWDM (dense wavelength division multiplex) systems to increase the bit rate. The wavelength channels are selected in a way that the information of the single channels can be selected at the receiver side and analyzed with an acceptable bit/error rate. Again, the bit rate is limited by the spectrum of the channels.
A modulation scheme know as VSB (vestigial side band modulation) is explained in xe2x80x9c5.12 Tbit/s Transmission over 3xc3x97100 km of Teralight fiberxe2x80x9d Bigo, S. et al., paperPD2, PP40-41, ECOC 2000.
The two side bands of a NRZ spectrum generally contain redundant information. It is therefore tempting to filter out one of them in order to increase spectral efficiency, a technique known as VSB. However VSB is difficult to implement at the transmitter because the suppressed side bands rapidly reconstruct through fiber non linearities.
So a VSB filtering at the receiver side is proposed. With a modulation and filtering scheme like VSB, the bandwidth efficiency increases to a value of more than 0.6 bit/s/Hz compared with 0.4 bit/s/Hz in conventional systems.
Again the transmission is limited due to the effects of cross talk between the adjacent channels.
The inventive solution comprises a VSB filtering scheme with alternating side band filtering and two sets of channels orthogonally polarized. The increase of bandwidth efficiency is important. The effects of cross talk between adjacent channels are minimized.
Each of these channels is generated by passing light into a modulator. The resulting optical spectrum consists of a carrier and two optical sidebands apart the carrier. The lower-wavelength sideband is referred to next as xe2x80x9cleft sidexe2x80x9d and the higher-wavelength side-band is referred to as xe2x80x9cright-sidexe2x80x9d. When sent into the transmission system, such spectrum is passed into a cascade of optical components with a filtering transfer function, such as wavelength division multiplexers, wavelength division demultiplexers, specific filters, etc. whether located within the transmitter or the receiver or in-line.
In the invention, the total set of modulated channels is divided into two frequency-interleaved subsets to be polarization-multiplexed with orthogonal polarizations. In each subset, the channel spacing is alternatively A and B, from one channel to another, with A less than B.
Furthermore, for each channel, the peak of the overall filter response (i.e. the product of all the filtering transfer functions along the system) is chosen so as to fall off the carrier frequency, such that the channels of each sub-set are alternatively left- and right-filtered. This is analogous to the vestigial-sideband (VSB) filtering technique, well-known to the radio engineers.